A. It is the purpose of this study to characterize endogenous oncornavirus-like particles found in association with cell cultures derived from human brain tumors and to determine their incidence and possible significance with respect to (a) associated cytogenetic abnormalities, (b) antigenic relationship to other oncornaviridae, viral and membrane antigens, (c) their infectivity and mode of proliferation. B. To study the function of the nucleolus in the transcription of cellular RNA's in avian sarcoma virus infection and the induction of the transformed state using E.M. autoradiography and polyacrylamide gel electrophoresis.